On the Activation screen, choose the option to Activate by phone.

    There's no limit to the number of times you can reinstall and
activate the same WinXP license on the same PC.  Nor is there ever a
charge.  Nor does a Product Key (so long as it's not an evaluation
license) ever expire.  If it's been more than 120 days  since you last
activated that specific Product Key, you'll most likely be able to
activate via the Internet without problem.  If it's been less, you
might have to make a 5 minute phone call.

    Here are the facts pertaining to activation:

Piracy Basics - Microsoft Product Activation
http://www.microsoft.com/piracy/basics/activation/

Windows Product Activation (WPA)
http://www.aumha.org/a/wpa.htm